I managed to implement it somewhat successful. I still have bullets passing through enemies, apparently not as many, though. So in order to understand what's going on, I have a question: By what conditions is a collision tested?

If a bullet collides with an enemy within the repeating pattern (say at 5th repetition out of 10), does that count as a collision? Because of this was the case, moving the bullet 1 pixel would always ensure a collision, right?

The reason I'm asking is because that's exactly what I've done, then tweak the bullet speed with the amount of repetitions per tick, but as I wrote before, there's still stray bullets.

You can use CustomMovement instead of Bullet for fast moving objects. Use the 'On CustomMovement step' trigger to check for collisions every step and adjust the 'pixels per step' setting to increase accuracy. With 'pixels per step' set to 1 you can check for collisions every pixel (though that would be overkill in most cases).